Premium
σ–π Energy Separation in Homodesmotic Reactions
Author(s) -
Hohlneicher Georg,
Packschies Lars,
Weber Johannes
Publication year - 2005
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500180
Subject(s) - aromaticity , antiaromaticity , chemistry , context (archaeology) , chemical physics , computational chemistry , conjugated system , partition (number theory) , molecule , organic chemistry , polymer , paleontology , mathematics , combinatorics , biology
A well‐established quantity for specifying the aromaticity or antiaromaticity of cyclic conjugated molecules is the so‐called aromatic stabilization energy (ASE), which can be derived—either experimentally or theoretically—from appropriate homodesmotic reactions. To gain further insight into the origin of aromaticity, several schemes have been devised to partition ASE into nuclear and electronic as well as σ and π contributions, some of which have resulted in contradictory statements about the driving force of aromatic stabilization. Currently, these contradictions have not been resolved and have resulted in a confusing distinction between two different types of aromaticity: extrinsic and intrinsic aromaticity. By investigating different homodesmotic reactions we show that, in contrast to ASE itself, the individual contributions that enter the ASE can strongly depend on the type of reaction. Caution is therefore advised if conclusions or physical interpretations are derived from the individual components. The contradictions result from the fact that some reactions suffer from an imbalance in the number of interaction terms at the two sides of the reaction equation. The concept of isointeractional reactions is introduced and results in the elimination of the imbalance. For these reactions, the contradictions disappear and the distinction between intrinsic and extrinsic aromaticity becomes unnecessary. As far as the σ–π partitioning is concerned, several schemes proposed in the literature are compared. Contradictory results are obtained depending on the partitioning scheme and reaction used. In this context, it is demonstrated that for the partitioning of the electron–electron interaction, the scheme introduced by Jug and Köster is the one that is most theoretically grounded.